I. Field of the Invention.
The present invention relates to techniques for processing of semiconductor films, and more particularly to techniques for processing semiconductor films on glass or other substrates.
II. Description of the Related Art.
Techniques for fabricating large grained single crystal or polycrystalline silicon thin films using sequential lateral solidification are known in the art. For example, in U.S. patent application Ser. No. 09/390,537, the contents of which are incorporated by reference herein and which application is assigned to the common assignee of the present application, particularly advantageous apparatus and methods for growing large grained polycrystalline or single crystal silicon structures using energy-controllable laser pulses and small-scale translation of a silicon sample to implement sequential lateral solidification are disclosed. Using the sequential lateral solidification technique, low defect density crystalline silicon films can be produced on those substrates that do not permit epitaxial regrowth, upon which high performance microelectronic devices can be fabricated.
The effectiveness with which sequential lateral solidification can be implemented depends on several factors, the most important of which corresponds to the length of lateral crystal growth achieved per laser pulse. Such lateral crystal growth depends on several parameters, including the duration of the laser pulses, film thickness, substrate temperature at the point of laser pulse irradiation, the energy density of the laser pulse incident on the substrate, and the effective thermal conductivity of the substrate. In particular, if all other factors are kept constant, reducing is the thermal conductivity of the substrate will have the effect of increasing lateral crystal growth.
While there have been attempts to utilize low thermal conductivity materials, such as porous glass, in connection with sequential lateral solidification for the purpose of enhancing lateral crystal growth, such attempts have not achieved commercially viable results. For example, when a porous glass layer is used under a silicon film in the sequential lateral solidification process densification, and subsequent physical distortion, of such glass has been observed. Accordingly, there exists a need in the art for a technique for fabricating substrates having a modified effective thermal conductivity in order to optimize the sequential lateral solidification process.
An object of the present invention is to provide substrates having modified effective thermal conductivity which can be later used in an optimized sequential lateral solidification process.
A further object of the present invention is to provide substrates having modified effective thermal conductivity.
Still a further object of the present invention is to provide substrates having a directionally optimized effective thermal conductivity.
Yet a further object of the present invention is to provide multi layer substrates where one or more of the subsurface layers act as a heat reservoir in order to optimize the effective thermal characteristics of the substrate.
In order to achieve these objectives as well as others that will become apparent with reference to the following specification, the present invention provides a substrate having modified effective thermal conductivity for use in the sequential lateral solidification process. The substrate includes a base layer, e.g., glass, a low conductivity layer formed adjacent to a surface of the base layer, a high conductivity layer formed adjacent to the low conductivity layer, and a silicon layer formed on the high conductivity layer.
In a preferred arrangement, the low conductivity layer is porous glass, and is in the range of 5,000 Angstroms to 2 microns thick. The high conductivity layer may be a metal, and should be sufficiently thin so as to not increase the overall vertical conductivity of the substrate, preferably in the range of 50 to 5,000 Angstroms thick.
An intermediate silicon compound layer is preferably formed between the silicon layer and the high conductivity layer. The silicon compound may be silicon dioxide, and should be sufficiently thick to prevent diffusion of impurities from the high conductivity layer. It is preferred that the silicon compound layer is in the range of 200 to 2,000 Angstroms thick.
In an alternative arrangement, the present invention provides a substrate having modified effective thermal conductivity for use in the sequential lateral solidification process, wherein the high conductivity layer is replaced by an internal subsurface melting layer. In this arrangement, the substrate includes a base layer, a low conductivity layer formed adjacent to the base layer, a subsurface melting layer having a melting point which is less than that of silicon and formed adjacent to the low conductivity layer, a silicon compound layer formed adjacent to the subsurface melting layer, and silicon layer formed on the silicon compound layer.